Several processes of the above general type are now well established in the art of gas purification with wide utility in the removal of H.sub.2 S from foul gas streams of various compositions and origins. The liquid reagent used differs in composition from process to process and application to application: in, for instance, the Stretford Process the liquid reagent is fundamentally an alkaline solution of one or more of the anthraquinone disulphonic acid (ADA) salts and contains also vanadium. The foul gas stream is brought into contact with this reagent, the alkali of which reacts with the H.sub.2 S in the gas to form the hydrosulphide which thereupon reacts both with the ADA and pentavalent vanadium, reducing the ADA to the hydroquinone form and the vanadium to the tetravalent state whilst liberating sulphur. The reagent is regenerated by blowing air through it, thereby floating the sulphur as a froth for removal and recovery and oxidising the reduced reagent materials back to their original respective form and state.
In the similar Takahax Process the liquid reagent is a solution of the sodium salt of napthaquinone disulphonic acid: a variant of this latter process is the Perox Process in which the liquid reagent is a solution of the ammonium salt of naphthaquinone disulphonic acid.
In practical operation of such processes, thiosulphate is formed as an unwanted by-product, as a result of oxidation of an initially-formed hydrosulphide. This oxidation may occur as a result of the presence of oxygen in the foul gas stream entering the reagent in the absorption stage or as a result of carry-over of unreacted hydrosulphide into the regeneration stage. In either event, the build-up of thiosulphate in the circulating reagent must be restricted. Similarly, sulphate tends to be formed in the reagent as an unwanted contaminant.
When, as is sometimes the case, the foul gas stream contains traces of HCN -- for instance when the foul gas is a coal carbonisation by-product -- this tends also to be absorbed by the circulating reagent and to appear as thiocyanate therein.
In conventional practice of these processes, the build-up of these unwanted contaminants in the circulating reagent is controlled by bleeding-off a small sidestream of spent reagent. There then arises the problem of disposal of the spent reagent, a problem that is especially acute when the contaminants include thiocyanate.